The invention relates to a display device for displaying a screen image comprising:
a first memory which in logic terms consists of at least two segments, each segment being adapted to store an image fragment made up of a plurality of fragment points,
image-building means for building the screen image from the image fragments or parts of the image fragments stored in the first memory, by consecutively reading a plurality of screen points corresponding to fragment points from the first memory,
The invention also relates to a method method of building up a screen image from at least two image fragments, each image fragment being stored in a memory segment which forms part of a first memory, an image fragment being made up of a plurality of fragment points, the display image being built up by consecutively reading a plurality of screen points corresponding to fragment points from the first memory.
Such a display device is known from EP-A 0 341 654. The known device is a map display device for use in an airplane and includes a bulk memory, a random access memory, a display memory, a data bus and a display screen. The bulk memory serves for the digital storage of maps in compressed form. The random access memory serves as a cache memory and is made up of segments capable of storing a random map-fragment in non-compressed form. Logically contiguous fragments then need not be stored in physically contiguous cache segments. The display memory stores the image being displayed on the display screen. If the image is to be changed the display memory is reloaded by composing the image from fragments stored in the cache memory. If a required fragment is not yet available from the cache memory it is read in from the bulk memory. The data bus serves for the transfer of data between the various components.
A disadvantage of the known device is that a special display memory is required and that there is extensive data traffic from the cache memory to the display memory and from the display memory to the screen. As a result of this, high demands are made on the speed of the required processors and the capacity of the data bus.
It is an object of the invention to provide a display device of the type defined in the opening paragraph which does not require a special display memory and which enables the occupancy of the data bus to be comparatively low.
To this end, the device in accordance with the invention is characterized in that the display device further comprises storage means for storing for each first segment pointing information to further segments, the image-building means being adapted to use the pointing information associated with a first segment to find a second segment, when two fragment points corresponding to consecutive screen points are contained in the first and the second segment respectively.
Owing to these measures the screen image can be composed directly from randomly arranged image fragments in the first memory. As long as fragment points are being read from a single segment, the address of a following fragment point can easily be derived from the memory address of the fragment point read in last. When a segment transition is encountered, which would normally cause an unacceptable delay, the segment containing the following fragment point can be located quickly by making use of the pointing information.
As a result of this, a special display memory is not necessary and the occupancy of the data bus is reduced considerably, enabling simpler components to be used or enabling the capacity which has thus become available to be used for other purposes.
The detection of segment transitions can be effected by means of storage registers and counters, preferably implemented in hardware, e.g. the registers containing segment dimensions and the counters being used for counting the number of fragment points read in from a particular segment. An additional advantage is that further process parameters, e.g. the dimensions of the display screen, can be stored in registers as well, thereby giving the designer freedom as regards the choice of components.
An embodiment of the device in accordance with the invention has the characteristic features defined in claim 2. It is customary to build up a line of a screen image by scanning pixels from one end of the line to the other end in a progressive way. If at some point of the screen line a segment transition is encountered, said pointer refers to the second segment which contains succeeding points of the screen line. In this way the scanning process proceeds without a substantial delay.
A further embodiment of the device in accordance with the invention has the characteristic features defined in claim 3. It is customary to build up a screen image by displaying lines from one end of the image to the other end in a progressive way, possibly in multiple cycles (interlaced scan). If at some screen line a segment transition is encountered, said pointer refers to the third segment which contains succeding lines of the screen image. In this way the scanning process proceeds without a substantial delay. A combination with the previous embodiment is very advantageous.
A further embodiment has the characteristic features defined in claim 4. An advantage of this embodiment is that the image in the first memory can also be displayed on the screen in a rotated form. A difference with the embodiment as claimed in claim 2 and claim 3 is that screen lines no longer have the same orientation as fragment lines, as a result of which a logically following screen point may correspond to, for example, a fragment point that forms part of a logically preceding fragment line. In this embodiment the pointing information of each cache segment therefore also comprises references to cache segments which store the logically preceding image fragment in a coordinate direction parallel to or perpendicular to the fragment lines. The projection means could, for example, be adapted to project the distance between two points of a rotated screen line onto said coordinate directions and determine the logically following fragment point by rounding to the nearest one. In a similar way the projection means could be applied in a direction perpendicular to the screen lines, e.g. for determining the fragment point which corresponds to the first point of a logically following screen line, given the fragment point corresponding to the first point of the preceding screen line.
The method in accordance with the invention is characterized in that for each segment pointing information to further segments is stored, using the pointing information associated with a first segment to find a second segment, when two fragment points corresponding to consecutive screen points are contained in the first and the second segment respectively.